If central banks can’t explain why the economic measure is so low, then why is there so much faith in their forecasts?

Bonds, equities and commodities appear to be sending contradictory messages about the economy, adding to the confusion from central banks struggling with the breakdown of their inflation models. However, the markets can be reconciled—and if they are right, the outlook is just dandy for Wall Street, if not so much for Main Street.

Unfortunately, the failure of central-bank models to predict the yearslong slowdown in global inflation leaves investors in the dark about the most important economic measure today and why it’s...